Method of production of parts of ahs steel by controlled local cooling by a cooling medium and by interrupted cooling at required temperature to create a multiphase microstructure

ABSTRACT

This invention generally relates to a method for producing parts of AHS steel via a controlled local cooling by a cooling medium and an interrupted cooling at a required temperature, without immersion in a cooling bath, to thereby create a multiphase microstructure. Typically, the steel part may be cooled by a jet of cooling medium so that, depending on the amount of heat which needs to be removed from the surface of the part, the locations from which a larger amount of heat needs to be removed are cooled at a higher intensity.

RELATED APPLICATIONS

This application claims the foreign priority benefit of Czech Patent Application Serial No. PV 2019-542 entitled “METHOD OF PRODUCTION OF PARTS OF AHS STEEL BY CONTROLLED LOCAL COOLING BY A COOLING MEDIUM AND BY INTERRUPTED COOLING AT REQUIRED TEMPERATURE TO CREATE A MULTIPHASE MICROSTRUCTURE,” filed Aug. 19, 2019, the entire disclosure of which is incorporated herein by reference.

FIELD OF INVENTION

This invention generally relates to a method for producing parts of Advanced High Strength (“AHS”) steel via controlled local cooling by a cooling medium and interrupted cooling at a required temperature, which creates a multiphase microstructure.

BACKGROUND ART

In the final stage of production, the structure of steel parts usually requires modification to attain the appropriate mechanical properties for engineering applications. The process which is often used to achieve this is a combination of quenching and tempering and is performed in various forms and with different parameters, depending on the material and the desired properties of the final structure. The typical structure after quenching is martensite, which exhibits high strength and hardness but very poor ductility owing to internal stress. This causes problems in parts under load, as they may suddenly fracture in service. Therefore, materials with a martensitic structure are usually tempered after quenching. Consequently, this produces a specific bainitic microstructure referred to as sorbite. Typically, sorbite consists of fine carbides and bainitic ferrite. Although this treatment sequence reduces the strength of the material somewhat, it also increases elongation, which is important to achieving the required operational safety of the part.

Recently, treatment processes were developed which can lead to elongation levels around 10%, even in multiphase martensitic structures, but their use is complicated as it requires that quenching is interrupted at defined temperatures, depending on the procedure. Interrupted cooling is typical of processing routes for modern AHS steels in which the microstructure is obtained by the quenching and partitioning process, long-time low-temperature austempering, or intercritical annealing for the TRIP effect. Typical temperatures at which quenching is interrupted lie in the 100 to 500° C. interval. This requires difficult quenching in various media, such as molten salts, which entails subsequent cleaning of the surface and disposal of waste. Consequently, these processes are costly and are an environmental burden. What is more, natural cooling and the non-uniform thickness of cross sections of products lead to non-uniform cooling rates, which can result in non-uniform microstructural evolution and internal stresses.

State of the art includes a solution described in Czech Patent No. CZ307645, which involves a method for producing steel parts by creating a multiphase microstructure that consists of low-stress martensite with increased ductility and stabilized retained austenite. This process utilizes a partial quenching in a quenching bath of boiling water, where the Ms (martensite start) temperature is above the boiling point of the quenching water medium and entails cooling of the steel semi-finished product from a temperature in the austenite region by repeated gradual immersion in and withdrawal from the quenching bath. After this, austenite stabilization is performed and the semi-finished product is then removed from a furnace and cooled to ambient temperature.

SUMMARY

One or more embodiments generally concern a method for producing AHS steel parts. Generally, the method involves a controlled local cooling by a cooling medium followed with an interrupted cooling at a required temperature to thereby create a multiphase microstructure. Furthermore, the steel part is cooled by a jet of cooling medium so that, depending on the amount of heat which needs to be removed from the surface of the steel part, the locations from which a larger amount of heat needs to be removed are cooled at higher intensity.

One or more embodiments generally concern a method for producing AHS steel parts. Generally, the method involves: (a) cooling of a steel part at controlled locations by a jet of cooling medium at a first temperature; and (b) cooling the steel part at a designated temperature that is different from the first temperature to thereby create a multiphase microstructure. Typically, the cooling of step (b) occurs at one or more intervals between the cooling of step (a). Furthermore, the first temperature of the cooling of step (a) may be adjusted depending on the amount of heat that needs to be removed from a surface of the steel part.

DETAILED DESCRIPTION

The above-listed drawbacks of today's production methods may be eliminated by a production method that is characterized by cooling of a semi-finished steel product from a temperature in the austenite region by a cooling medium supplied via nozzles in such a manner that, depending on the amount of heat required to be removed through the surface of the semi-finished product, the cooling intensity is adjusted so that higher-intensity cooling is performed in locations from which larger amounts of heat need to be removed. In one or more embodiments, this can be achieved by shaped nozzles that deliver the cooling medium in a controlled fashion to locations which must be cooled at a certain intensity. Additionally or alternatively, in various embodiments, this can be achieved by cooling with a system of nozzles arranged to deliver the cooling medium to required locations. Additionally or alternatively, in other embodiments, this may be achieved by supplying the cooling medium to the semi-finished steel product at a uniform intensity per unit of area with shrouds placed between the nozzles and the semi-finished product in order to shield off locations with less accumulated heat so that higher-intensity cooling takes place in locations without the shrouds. In such embodiments, these shrouds can be changed during the process, depending on changes in the temperature field or on the requirements for temperature gradients. Additionally or alternatively, in yet other embodiments, this may be achieved by a matrix of controlled nozzles producing a uniform temperature field in the semi-finished steel product as possible. Additionally or alternatively, in still yet other embodiments, this can be achieved by nozzles movable by means of a robot or another manipulator to thereby cool the semi-finished steel product contour gradually, while increasing the cooling intensity accordingly in pertinent locations by either raising the pressure of the medium, slowing down the movement of the nozzle across the location, passing the nozzle repeatedly over the location, or bringing the nozzle closer to the semi-finished product.

In various embodiments, the cooling system and its control are arranged to ensure that the temperature at locations with the least amount of accumulated heat, such as thin walls, ribs, or edges, does not decrease below the required cooling temperature of the semi-finished steel product.

In one or more embodiments, after forging, trimming, and sizing, a forged part is gripped in robot grippers. Based on a defined cooling sequence, the robot may be programmed to move the forged part in front of a nozzle, which provides a constant cooling intensity, in order to cool locations with the highest temperature. In such embodiments, the temperature may be monitored by an infrared (IR) camera. During the process, the camera system may identify the location with the highest temperature and a control algorithm may cause the robot to place the forged part in front of the nozzle in a way that leads to the smallest temperature gradients on the surface being cooled. Once certain temperatures are reached in a defined region, the heat conducted from the core of the forged part ceases to increase the surface temperature, and the temperature begins to decrease even when no medium is supplied from the nozzles, the cooling process is finished and the robot passes the forged part on to the next operation.

In one or more embodiments, a flat machined semi-finished steel product heated to 850° C. is placed in a fixture, which maintains it in an exactly defined position. Pipes may be provided on both sides of the semi-finished steel product, which are shaped to follow the contour of the semi-finished product at locations with the thickest walls. In such embodiments, nozzles may be attached to the pipes, wherein the size and shape of the nozzles may be adjusted to provide the required cooling intensity. The supply of the cooling medium at the pipe inlet may be controlled by a valve in a manner that leads to the removal of as much energy from the thicker portion of the semi-finished product as possible without letting the temperature decrease below a required value. When the surface temperature becomes close to the required temperature, the process is interrupted by the valve and the process only resumes once the temperature increases again due to the heat conducted from the interior of the semi-finished product. After the surface and the interior temperatures equalize, when an interruption does not lead to an appreciable increase in the surface temperature, the process is ended. In such embodiments, the portions of the semi-finished product with less thickness may be cooled simultaneously, which makes the process faster.

In one or more embodiments, a rotation-symmetric multiple-stepped shaft at 910° C. is placed in a fixture, which allows it to rotate. On a linear guideway, a nozzle may be positioned along the shaft in a direction parallel to its axis, where the digital control of the nozzle enables it to deliver the required amounts of the cooling medium in pulses. The cooling intensity may be controlled by the speed of movement of the nozzle and by its opening and closing sequences. Depending on the diameter of the shaft, the cooling medium removes heat energy at various intensities, gradually reducing the temperature of the shaft to the desired value, while maintaining approximately constant temperature in all parts of the surface of the shaft. This minimizes any stresses generated due to thermal and transformation dilatation.

This invention can find broad use in heat treatments and thermomechanical treatments of blanks from high-strength steels, namely in production of AHS steel parts, typically for the machinery and transport industry.

Definitions

It should be understood that the following is not intended to be an exclusive list of defined terms. Other definitions may be provided in the foregoing description, such as, for example, when accompanying the use of a defined term in context.

As used herein, the terms “a,” “an,” and “the” mean one or more.

As used herein, the term “and/or,” when used in a list of two or more items, means that any one of the listed items can be employed by itself or any combination of two or more of the listed items can be employed. For example, if a composition is described as containing components A, B, and/or C, the composition can contain A alone; B alone; C alone; A and B in combination; A and C in combination, B and C in combination; or A, B, and C in combination.

As used herein, the terms “comprising,” “comprises,” and “comprise” are open-ended transition terms used to transition from a subject recited before the term to one or more elements recited after the term, where the element or elements listed after the transition term are not necessarily the only elements that make up the subject.

As used herein, the terms “having,” “has,” and “have” have the same open-ended meaning as “comprising,” “comprises,” and “comprise” provided above.

As used herein, the terms “including,” “include,” and “included” have the same open-ended meaning as “comprising,” “comprises,” and “comprise” provided above.

Numerical Ranges

The present description uses numerical ranges to quantify certain parameters relating to the invention. It should be understood that when numerical ranges are provided, such ranges are to be construed as providing literal support for claim limitations that only recite the lower value of the range as well as claim limitations that only recite the upper value of the range. For example, a disclosed numerical range of 10 to 100 provides literal support for a claim reciting “greater than 10” (with no upper bounds) and a claim reciting “less than 100” (with no lower bounds).

CLAIMS NOT LIMITED TO DISCLOSED EMBODIMENTS

The preferred forms of the invention described above are to be used as illustration only, and should not be used in a limiting sense to interpret the scope of the present invention. Modifications to the exemplary embodiments, set forth above, could be readily made by those skilled in the art without departing from the spirit of the present invention.

The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as it pertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims. 

What is claimed is:
 1. A method for producing AHS steel parts, the method comprising: (a) cooling of a steel part at controlled locations by a jet of cooling medium at a first temperature; and (b) cooling the steel part at a designated temperature that is different from the first temperature to thereby create a multiphase microstructure, wherein the cooling of step (b) occurs at one or more intervals between the cooling of step (a), wherein the first temperature of the cooling of step (a) may be adjusted depending on the amount of heat that needs to be removed from a surface of the steel part.
 2. The method of according to claim 1, wherein thin regions of the steel part that contain less steel material relative to thicker regions of the steel part that contain more steel material are cooled in a controlled fashion during the cooling of step (a) and the cooling of step (b) so that an undercooling temperature in the thin regions is higher than a minimum required cooling temperature.
 3. The method according to claim 1, wherein a uniform temperature is maintained on the surface of the steel part.
 4. The method according to claim 1, wherein the cooling of step (a) and the cooling of step (b) are affected by a nozzle. 